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used to reconstruct the historical data are cited within these modern publications.

Analyzing Change in Naturalized Richness

Data on change in naturalized richness through time (Appendix) were analyzed in multiple ways. First, data were plotted on individual islands over time (Fig. 5.3). Second, the mean ratio of naturalized to native richness was calculated in 20-year intervals over the past 140 years (Table 5.1); e.g., the penultimate interval, 1980, extended from 1971 to 1990. Native richness was held constant across these ratio analyses, using the currently recorded values (Appendix Table 5A.1); this was done because the actual number of native species on islands has changed very little over the past 140 years, because few species have been lost and presumably few native species gained during this time period. Naturalized richness values were taken from the Appendix. When more than one date was available from a single 20-year time interval, then the date closest to the midpoint of the time period was used; if two dates were equally close to the midpoint, then the one closest to dates analyzed for other islands in that time period was used. Third, these same native and naturalized values were plotted by 20-year time intervals (Fig. 5.4).

Note that the last date recorded for any individual island is subject to a procedural artifact that reduces the total number of naturalized species. This artifact lowers the number of naturalized species not just for individual islands, but also for mean estimates of naturalized species across islands. This procedural artifact occurs because exotic species are only counted as “naturalized” if they are seen (i) in multiple places on an island within a single time step or (ii) within the same area across multiple time steps, i.e., across two or more survey periods that are separated sufficiently for at least one turnover in generation time to have occurred [see Sax et al. (2002) for a full discussion]. Consequently, species seen in only one place on an island and recorded from only the most recent island survey will not be considered to be naturalized unless they persist to the next survey period, when the number of naturalized species from the previous time period would then be revised upward.

ACKNOWLEDGMENTS

We thank Jay Stachowicz and members of the National Center for Ecological Analysis and Synthesis Working Group for valuable comments on the data discussed in this manuscript and Kate Smith, John Avise, and two anonymous reviewers for valuable comments that improved this manuscript. We thank the organizers of the Sackler Colloquium, In the



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